A British submarine has joined the search for the missing Malaysia Airlines plane on Wednesday as crews search under, on and above the surface of the ocean for any sign of the aircraft.

HMS Tireless will assist a fleet of aircraft from six nations and numerous ships already involved in the search 1,800 kilometres south-west of Perth.

The Boeing 777 is believed to have crashed in the southern Indian Ocean 25 days ago with 239 people on board, after flying thousands of kilometres off course.

Malaysia's prime minister Najib Razak has arrived in Perth to be briefed on the search and meet with those involved in the mission.

The head of the search operation, former Australian Defence Force Air Chief Marshal Angus Houston, says the mission is one of the most difficult ever undertaken anywhere in the world and it will continue for some time.

"I can't think of a set of circumstances in a search and recovery as demanding and as complex as this," he said on Wednesday.

"We have to keep searching and searching with vigour, because what's really vital here is to find some wreckage, some debris on the surface of the ocean.

"It's only through that that we'll hopefully lead to where the aircraft is on the bottom of the ocean."

He says a lack of detailed information about the plane's final movements makes the search extremely difficult.

"Over time if we don't find anything on the surface we are going to have to think about what we do next."

The final entry from just after 1:19am comes from one of the two MH370 pilots, who says "good night, Malaysian three seven zero".

The plane disappeared from radar shortly thereafter and is yet to be found, though Malaysian investigators believe it was deliberately diverted and flew on for hours to the Indian Ocean, where it is presumed to have crashed.

The transcript - and particularly the final words from MH370 - has been the subject of much speculation following earlier statements by authorities and the airline that the last transmission from the plane was a casual "All right, good night".

That apparent non-standard sign-off fuelled speculation that one of the pilots - either captain Zaharie Ahmad Shah, 53, or first officer Fariq Abdul Hamid, 27 - diverted the plane on purpose.

Under pressure to clarify the matter, the Malaysian government corrected the final words late on Monday, but did not release a full transcript.

A statement from Malaysia's acting transport minister, Hishammuddin Hussein, said the transcript was "initially held as part of the police investigation", but gave no other reason for the delay.

Malaysia Airlines had said previously that the last words were believed uttered by Mr Fariq, but the statement said the ongoing investigation was yet to confirm that.

Mr Hishammuddin has repeatedly denied various anonymously sourced reports revealing details of Malaysia's investigation into the disappearance of MH370.

The transport minister took particular aim at reports published by Britain's Daily Mail concerning Mr Zaharie.

The tabloid quoted a "source close to the family" of the pilot as saying police had learned he was emotionally unstable before the flight amid alleged marital trouble.

"I can confirm to you that the information did not come from the police and you should ask Daily Mail how they got the information," Mr Hishammuddin said.

The Daily Mail also reported earlier that Mr Zaharie was said to be a fanatical supporter of Malaysia's political opposition.

Friends and acquaintances have denied the allegation.

Nine ships and 10 aircraft resumed the hunt for wreckage on Tuesday, hoping to recover more than the fishing gear and other flotsam found since authorities moved the search 1,100 kilometres north after new analysis of radar and satellite data.